1. Field of the Invention
The present invention relates to a dielectric filter formed by providing electrodes on both inside and outside of a dielectric block, also relates to a duplexer and a communication apparatus fabricated by using the above dielectric filter and the duplexer.
2. Description of the Related Art
Several dielectric filters each formed by providing a plurality of resonators in a dielectric block may be classified into several types, and are used in different ways in accordance with different purposes. As one type of an dielectric filter in which one end face of the dielectric block is used as an open face while an opposite end face is used as a short circuit face, there have been several patent publications which disclose the following arrangements.
(1) Japanese Unexamined Patent Publication No. 6-310911 discloses that electrically conductive materials are disposed in all the surfaces and through holes except the first end face, cross sectional shape of the through holes on the first end face side are made different from that on the second end face side, so that the same resonators are combined together with the characteristic impedance on one side being different from the other.
(2) Japanese Unexamined Utility Model Publication No. 63-181002 discloses that a plurality of through holes are arranged in parallel with one another in a dielectric block, and a coupling hole is provided between these through holes.
(3) U.S. Pat. No. 5,146,193 (Japanese Unexamined Patent Publication No. 6-505608) discloses that through holes having constant cross sections are provided in a dielectric block, and input/output electrodes capable of surface mounting are provided on the side face of the dielectric block.
(4) Japanese Unexamined Patent Publication No. 7-86807 discloses that a plurality of through holes are formed in a dielectric block, one end of each through hole is formed into a short circuit face while the other end thereof is formed into an open face. Further, a recess portion is formed on the open face side; and a conductor is disposed in the recess portion for getting in connection with conductors within the through holes, thereby forming a desired load capacity.
However, with the dielectric filters of the above (1), (2) and (3), when a dielectric block having a pure rectangular parallelepiped shape is used, there is a problem that it is difficult to obtain a large freedom in designing for obtaining a desired characteristic. Moreover, there is no attenuation pole existing in the frequency gain property.
With the dielectric filter of the above (4), the load capacity is created and resonator length is shortened by forming a recess portion on open end face side of each through hole.
Preferred embodiments of the present invention provide a dielectric filter, a duplexer and a communication apparatus fabricated by using the dielectric filter and the duplexer, all free from the above-discussed problems.
In order to control a coupling coefficient between resonators and to control an attenuation pole frequency, a distance (pitch) between through holes adjacent to each other and a difference (a step ratio) between cross section sizes of the through holes are used as structural parameters, thereby making it possible to change a self-capacitance and a mutual capacitance, both of which may be calculated with the use of the above parameters.
However, in a prior art dielectric filter, since the cross sections of through holes extending from a first end face side to a second end face side are all circular, it is impossible to obtain a necessary capacity value of Cij and a necessary coupling coefficient, all within a range allowed by realizable formation sizes. Further, in order to form an attenuation pole at a frequency in the vicinity of passing frequency band, it is necessary to increase both Cij on the open face side and Cij on the short circuit face side. But, since it is impossible to obtain a necessary capacity value of Cij within a range allowed by realizable formation sizes, it is probably impossible to form an attenuation pole at a desired frequency.
Here, the present invention is a dielectric filter wherein a plurality of through holes are formed in a dielectric block having a generally rectangular parallelepiped shape, extending from a first end face thereof and reaching a second end face thereof opposite to the first end face, internal conductors are provided within the holes, a conductor with its first end face serving as an open face and its second end face serving as a short circuit face is provided on the external surface of the dielectric block, thereby forming a plurality of dielectric resonators, cross sections of the holes are made rectangular, a depth which is 10% to 50% of the hole length extending from the first end face to the second end face is served as a step position, an internal size of each hole extending from the step position to the first end face is different from an internal size of the hole extending from the step position to the second end face.
In this way, since cross sections of the holes formed in the dielectric block are made rectangular, a depth which is 10% to 50% of the hole length extending from the open face is served as a step position, if compared with a case where the cross sectional shape of the through holes are circular, it is possible that Cij may be made large when the coupling coefficient has been made constant. Thus, an attenuation pole may be generated in a position close to a central frequency of a passing frequency band, thereby making it possible to improve a characteristic of an attenuation amount in the vicinity of a passing frequency band.
Further, the present invention is a dielectric filter wherein a plurality of through holes are formed in a dielectric block having a generally rectangular parallelepiped shape, extending from a first end face thereof and reaching a second end face thereof opposite to the first end face, internal conductors are provided within the holes, a conductor with its first end face serving as an open face and its second end face serving as a short circuit face is provided on the external surface of the dielectric block, thereby forming a plurality of dielectric resonators, cross sections of the holes are made rectangular, a depth which is 10% to 50% of the hole length extending from the first end face to the second end face is served as a step position, a cross section of each hole extending from the step position to the first end face is rectangular, and a cross section of the hole extending from the step position to the second end face is circular.
In this way, since the cross sections of the through holes on the short circuit face side having a high electric current density are made into circular shapes, it is possible to avoid a current concentration on top portions of through holes having square cross section, thereby enabling the electric current to be distributed uniformly and thus improving a no-load Q (Qo). Further, since the cross sections of the through holes on the open face side are formed into rectangular shape, it is allowed to increase a designing freedom for designing the above Ci and Cij, thus making it possible that an attenuation pole may be generated in a position closer to a central frequency of a passing frequency band.
Further, according to the present invention, there is provided a duplexer having a plurality of dielectric filters, characterized in that at least one group of dielectric filters are incorporated in a single one dielectric block. In this manner, it is possible to obtain a duplexer comprising a reception filter capable of attenuating a transmission frequency band and allowing the passing of reception frequency band, and a transmission filter capable of attenuating a reception frequency band and allowing the passing of transmission frequency band. Therefore, since it is possible to generate an attenuation pole frequency in the vicinity of a passing frequency band, the above reception filter and the above transmission filter are suitable for use in a case where a transmission frequency band and a reception frequency band are close to each other.
Further, according to the present invention there is provided a communication apparatus characterized in that said device is formed by providing in its high frequency circuit either a dielectric filter or a duplexer. Therefore, it is possible to obtain a communication apparatus which is compact in size and has an excellent characteristic such as an excellent CN ratio of a high frequency circuit section.